Re: [PATCH v2 1/2] dma/iommu: Add pgsize_bitmap confirmation in __iommu_dma_alloc_pages

[Doug Anderson]

Will,

On Tue, Mar 29, 2016 at 10:02 AM, Will Deacon <will.deacon@xxxxxxx> wrote:
> On Mon, Mar 28, 2016 at 02:32:11PM +0800, Yong Wu wrote:
>> Currently __iommu_dma_alloc_pages assumes that all the IOMMU support
>> the granule of PAGE_SIZE. It call alloc_page to try allocating memory
>> in the last time. Fortunately the mininum pagesize in all the
>> current IOMMU is SZ_4K, so this works well.
>>
>> But there may be a case in which the mininum granule of IOMMU may be
>> larger than PAGE_SIZE, then it will abort as the IOMMU cann't map the
>> discontinuous memory within a granule. For example, the pgsize_bitmap
>> of the IOMMU only has SZ_16K while the PAGE_SIZE is SZ_4K, then we
>> have to prepare SZ_16K continuous memory at least for each a granule
>> iommu mapping.
>
> Did you find a driver/platform that actually needs this? I certainly
> think it's possible, but we don't necessarily need to fix it if it's
> not broken yet! Just adding a comment would be enough.

I think Yong Wu was adding this patch in response to your request from
v1 of <https://patchwork.kernel.org/patch/8487511/>.  Fixing this
seemed like a separate issue so I asked Yong Wu to make two patches
rather than sticking this fix in the patch as his original.  Hopefully
this makes sense.

> Either way, a couple of review comments inline.
>
>>
>> Signed-off-by: Yong Wu <yong.wu@xxxxxxxxxxxx>
>>
>> ---
>> v2:
>>  -rebase on v4.6-rc1.
>>  -add a new patch([1/2] add pgsize_bitmap) here.
>>
>>  drivers/iommu/dma-iommu.c | 22 +++++++++++++---------
>>  1 file changed, 13 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
>> index 72d6182..75ce71e 100644
>> --- a/drivers/iommu/dma-iommu.c
>> +++ b/drivers/iommu/dma-iommu.c
>> @@ -190,11 +190,13 @@ static void __iommu_dma_free_pages(struct page **pages, int count)
>>       kvfree(pages);
>>  }
>>
>> -static struct page **__iommu_dma_alloc_pages(unsigned int count, gfp_t gfp)
>> +static struct page **__iommu_dma_alloc_pages(unsigned int count, gfp_t gfp,
>> +                                          unsigned long pgsize_bitmap)
>>  {
>>       struct page **pages;
>>       unsigned int i = 0, array_size = count * sizeof(*pages);
>> -     unsigned int order = MAX_ORDER;
>> +     int min_order = get_order(1 << __ffs(pgsize_bitmap));
>
> 1UL

Yong: presumably you will spin with this fix?


>> +     int order = MAX_ORDER;
>>
>>       if (array_size <= PAGE_SIZE)
>>               pages = kzalloc(array_size, GFP_KERNEL);
>> @@ -213,13 +215,16 @@ static struct page **__iommu_dma_alloc_pages(unsigned int count, gfp_t gfp)
>>               /*
>>                * Higher-order allocations are a convenience rather
>>                * than a necessity, hence using __GFP_NORETRY until
>> -              * falling back to single-page allocations.
>> +              * falling back to min size allocations.
>>                */
>> -             for (order = min_t(unsigned int, order, __fls(count));
>> -                  order > 0; order--) {
>> -                     page = alloc_pages(gfp | __GFP_NORETRY, order);
>> +             for (order = min_t(int, order, __fls(count));
>> +                  order >= min_order; order--) {
>> +                     page = alloc_pages((order == min_order) ? gfp :
>> +                                        gfp | __GFP_NORETRY, order);
>>                       if (!page)
>>                               continue;
>> +                     if (!order)
>> +                             break;
>
> Isn't this handled by the loop condition?

He changed the loop condition to be ">= min_order" instead of "> 0",
so now we can get here with an order == 0.  This makes sense because
when min_order is not 0 you still want to run the code to split the
pages and it is sane not to duplicate that below.

Maybe I'm misunderstanding, though.  Perhaps you can explain how you
think this code should look?

-Doug